This invention relates generally to continuous fluid level measurement systems and, more particularly, to a continuous fluid level measuring system that has a capacitance probe for providing more reliable and accurate level measurements.
One common fluid level measuring system of this particular kind utilizes a capacitance probe having an electrode mounted within a grounded metal container that can be filled with a liquid or the like. The grounded container functions as a second electrode, spaced apart from the probe electrode, to form a capacitor. The capacitance of the probe varies with the container's shape, the location of the probe electrode in the container, and the dielectric constant of the insulating material between the probe electrode and the container. Since the dielectric constant for the liquid is typically much larger than the dielectric constant for air, the capacitance between the electrode and the container continuously changes as the liquid level changes, the liquid acting as a variable-thickness dielectric. The capacitance between the probe electrode and the container electrode thus can provide an indication of the liquid level in the container.
Another common continuous capacitance probe has two elongated insulated electrodes placed upright within a container. The capacitance between the two electrodes changes as the liquid level changes. Although insulated, such capacitance probes are directly connected to grounded measurement circuits, which are susceptible to ground currents induced on the probe or connecting wires.
The capacitance of these probes may be measured using the circuit shown in FIG. 1. The capacitance probe Cx is connected between the inverting input terminal (-) of an operational amplifier U1 and a ground terminal. An oscillator G1 is connected between the operational amplifier's noninverting input terminal (+) and the ground terminal. A feedback capacitor C1 is connected between the operational amplifier's output terminal and its inverting input terminal (-). A constant amplitude ac voltage signal Vs from the oscillator is applied between the noninverting input terminal and the ground terminal. As a result, the operational amplifier, through the feedback capacitor, forces the voltage at the inverting input terminal to track the voltage signal Vs at noninverting input terminal. This voltage at the inverting input terminal induces a probe current through the capacitance probe that is related to the probe's capacitance. At the output terminal, the amplifier sets an output voltage Vo to cause a feedback current, which is substantially equal to the probe current, to flow through the feedback capacitor. Thus, the amplifier's output voltage Vo is proportional to the probe's capacitance. Accordingly, as the liquid level increases, the capacitance of the probe increases, causing a corresponding increase in the amplitude of the output signal.
Generally, the accuracy of the continuous liquid level measuring system is limited by the resolution of the capacitance probe and related circuitry. However, when a capacitance probe is used with a highly ionic or viscous liquid, the liquid tends to coat the probe even at low levels, which can cause inaccurate measurements. Attempted solutions, such as covering the probe with Teflon, have not eliminated the coating problem. Other solutions for the coating problem, such as guard shields, have required a grounded container to act as one of the probe's electrodes, thereby preventing the use of an ungrounded probe and limiting the probe's geometry.
From the discussion above, it should be apparent that there is a need for a continuous liquid level measurement system having a totally isolated capacitance probe that is unaffected by either a viscous conductive liquid coating or an ungrounded container. Likewise, there is a need for a capacitance measurement circuit that provides a totally isolated connection to the isolated capacitance probe. The present invention satisfies these needs.